Electro hydraulic motors

ABSTRACT

An electro-hydraulic motor is described in which a rotary hydraulic motor is slaved to a stepping pilot motor by the operation of a fluid distributor valve which is controlled by a differential device responsive to the angular discrepancy between the shafts of the pilot and hydraulic motor. In this invention, the differential device consists of a toothed wheel coupled to the electric pilot motor to drive a shaft which is slidable coaxially and rotatable relative to the hydraulic motor shaft: a frontal cam carried by the latter shaft cooperates with a roller mounted on a face of the toothed wheel to cause proportional axial displacement of the differential shaft, which is transmitted to the spool of the distributor valve, preferably mounted at right angles to the hydraulic motor shaft.

O Unlted States Patent 1 1 1 1 3,721,158

4 DAddea et a1. ]March 20, 1973 ELECTRO HYDRAULIC MOTORS 3,583,2866/1971 Chiappulini..-. ..9l/491 inventors: NllZiO DAddea, P Panzen,Macherlo; Ruggero Chiap- I pulini, Sesto San Giovan i all r 832,8539/1949 Germany ..9l/481 ofltaly 978,514 11/1950 France ..91 3s [73]Assignee: Consiglio Nazionale Delle Ricerche, Primary Examiner wmiam LFreeh Rome Italy Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak [22]Filed: Feb. 2, 1971 57 [21] Appl. No.: 112,000 1 ABSTRACT Anelectro-hydraulic 'motor is described in which a rotary hydraulic motoris slaved to a stepping pilot [301 Fol-mg Apphcamm Pnomy Data motor bythe operation of a fluid distributor valve Feb. 6, 1970 Italy ..67382A/70 which is lled by a dif erential device responsive to the angulardiscrepancy between the shafts of the [52] US. Cl ..91/35, 91/481 piloand hydraulic motor. In this invention, the dif- [51] Int. Cl ..F15b21/02 fer ntial device consists of a t othed wheel coupled to [58] Fieldof Search ..91/35, 39, 491, 481 the electric pilot motor to drive ashaft which is slidable coaxially and rotatable relative to thehydraulic [56] References Cited motor shaft: a frontal cam carried bythe latter shaft cooperates with a roller mounted on a face of theUNITED STATES PATENTS toothed wheel to cause proportional axialdisplace- 613,124 10/1898 Chagnaud ..91/481 ment of the differentialShaft, which is transmitted to 2,952,219 9/1960 Woydt ..91/481 the spoolof the distributor valve, Preferably mounted- 3,0l6,882 111962 Marco..91/39 at right angles to the hydraulic motor shaft 3,179,016 4/1965Thornton-Trump ..9l/481 33056387 10/1962 Budzich ..91/35 10 Claims,Z'DraWmg Figuresv 3,396,633 8/1968 Ryzner ..91/481 2a 47 45 as 42 1PATENTEDMARZO I975 33. 721, 158

SHEET 10F 2 INVENTORS NU/YZ/O D 'ADDEA (Es/1R5 PAP/25K! AuqqfkoCH/APPULl/Y/ Y yu m, mm m,

L ma r M ATTORNEYS PATENTEDHARZO ms SHEET 2 OF 2 ELECTRO HYDRAULICMOTORS This invention relates to electro-hydraulic motors forcontrolling the movement of slides and the like in machine tools,particularly machine tools having numerical control.

More specifically, the invention relates to an electrohydraulic motor ofthe type comprising a stepping electric pilot motor, a rotary hydraulicpower motor, a distributor valve for regulating the supply of workingfluid to the hydraulic motor and a differential device adapted todisplace a movable stem of the distributor valve proportionally to theangular discrepancy existing between the shaft of the electric pilotmotor and the shaft of the hydraulic motor.

Electro-hydraulic motors of this type are very useful for operatingslides in machine tools under numerical control. Such motors can be usedas angular position servos and can control machine tools in a simpleopen loop. As a consequence the use of such motors avoids the necessity,in machine tool control systems, for feed back means, generally formedby a position indicator independently detecting the position of theslide, and by an associated feedback loop.

In general known electro-hydraulic motors include, as already stated, astepping electric pilot motor which converts the electrical digitalincoming impulses into angular steps. The angular steps of the pilotmotor are used as a mechanical control input to a rotary hydraulicservo-system which operates in a closed control loop and which suppliesa torque output sufficiently amplified to carry out the desired movementof the slide or other moving member controlled by the electrohydraulicmotor in the machine tool.

The hydraulic servo-system comprises a rotary hydraulic power motor anda distributor valve which controls the supply and exhaust of the workingfluid of the motor. The distributor valve has a stem which is parallelto the shaft of the hydraulic motor and is connected both to the shaftof the electric pilot motor and to the shaft of the hydraulic motor.

In such known systems the distributor valve stem usually rotates aboutits axis with an angular speed proportional to that of the electricpilot motor, according to a fixed transmission ratio and is connected tothe shaft of the hydraulic motor by means of a screw coupling (i.e., thestem of the distributor valve being threaded and screwing into a nuthoused in the valve body, rotation of the valve stem causing the stem tomove axially relatively to the nut). The screw coupling forms adifferential device which causes the hydraulic servo system to effectaxial displacement of the distributor valve stem in proportion to thedifference existing at any given moment between the angular positions ofthe distributor (and, therefore, of the shaft of the electric pilotmotor) and the shaft of the hydraulic motor. The distributor thereforecontrols the supply of working fluid to the hydraulic motor to cause thelatter to follow constantly the movement of the electric pilot motor.

The known electro-hydraulic motors of the above mentioned type havenumerous disadvantages.

The distributor valve is generally disposed with its axis parallel tothe axis of the hydraulic motor. In consequence, even if the valve isdirectly supported by the outside body of the hydraulic motor, conduitsof a certain length are necessary to interconnect hydraulically thedistributor valve and the hydraulic motor. The variation of volume ofthe fluid contained in the said conduits during the operation of thehydraulic motor and the dilation of these conduits under pressure lessenthe overall rigidity of the system, increase its time constants anddistort the output. Moreover this positioning of the distributor valverelative to the hydraulic motor makes the whole electro-hydraulic motorbulky and difficult to mount on a machine tool.

The fact that the distributor valve stem rotates on its own axis insidethe casing of the distributor valve causes a rapid wear on the valveitself.

Finally, the use of a differential device formed by a screw couplingcauses difficulties both in the construction of the differential deviceitself and in replacement thereof should it be desired to vary the gainof the hydraulic servo system. The mating male and female parts of thescrew coupling must be manufactured to a high degree of precision foraccurate control, and this makes for high production costs, In order tovary the gain of a servo system having this type of differential devicethe pitch of the screw coupling must be charged. It is thereforenecessary to provide an electro-hydraulic motor of this type with aseries of screw-type differential devices, of different pitch, each ofwhich is in itself of high cost.

An object of this invention is to minimize the above mentioneddisadvantages in an electro-hydraulic motor of the type stated.

It is a further object to provide an electro-hydraulic motor of highefficiency and precision, with good accessibility to facilitateconstruction, servicing and repair.

According to the invention there is therefore provided anelectro-hydraulic motor of the type stated, characterized in that thedifferential device is formed by a toothed wheel coupled directly to theelectric pilot motor and fitted on a shaft coaxial with the shaft of thehydraulic motor, means supporting the said shaft for rotation and axialdisplacement relative to the hydraulic motor shaft, a frontal camrotatable with the latter shaft and facing the toothed wheel, andcam-follower means interposed between the frontal cam and the toothedwheel and supported by the latter; and further characterized in that thedistributor valve is placed transversely to the shaft of the hydraulicmotor and is directly connected to the latter through the interpositionof a variable shock-absorbing laminar member so that the distributorvalve is connected hydraulically to respective supply/exhaust conduitsof the hydraulic motor, transmitting means being provided between theshaft of the differential device and the stern of the distributor valveto transform the axial movements, in either sense, of the said shaftinto corresponding axial movements of the stem, and elastic means beingprovided to maintain said shaft and the stem always in contact with thesaid transmitting means.

Further characteristics and advantages of this invention will beapparent from the following description, given by way of non-limitingexample, with reference to the accompanying drawings, in which:

FIG. 1 is a side elevational view, partially in section, of anelectro-hydraulic motor according to one embodiment of the invention,and

FIG. 2 is an axial section, on an enlarged scale, of part of the motorof FIG. 1.

Reference numeral 1 indicates generally an electrohydraulic motorcomprising an hydraulic drive motor 2, an electric pilot stepping motor3, a distributor valve 4 hydraulically connected to the hydraulic motor2, and a differential device 5 mechanically interconnecting thehydraulic motor 2 and the distributor valve 4.

The hydraulic motor 2 is of the type described and claimed in US. Pat.No. 3,583,286.

The hydraulic motor 2 has a substantially cylindrical shape and isformed by two parallel circular plates 6 and 7 between which an annularbody 8 is interposed.

The annular body 8 has a number of radial bores 9 distributed uniformelyaround the body 8 and each housing a sleeve 10 in which a piston 11 ismounted for fluid-tight sliding movement. Each sleeve 10 is providedwith a number of axially spaced circumferential annular grooves 12, 13,14 and 15, each of which communicates with respective tangentialconduits indicated diagrammatically at16, 17, 18 and 19.

The annular body 8 is provided with two internal annular conduits and21, extending over the entire circumference of the body 8, which are indirect communication with the tangential conduits 19 and 17 respectivelyand which are employed alternatively, according to the sense of rotationof the hydraulic motor 2, as inlets and exhaust passages for workingfluid under pressure. To this end the annular conduits 20 and 21 in turncommunicate with respective conduits 22 and 23 in the plate 7, leadingto respective ports on the outside of the hydraulic motor 2.

Each piston 11 is connected at its radially inner end, and through aspherical coupling 24, to a stirrup 25 which supports a roller 26 forrotation about an axis parallel to the axis of the annular body 8. Eachroller 26 is in rolling contact with a twin-lobe cam 27 which isintegral with a shaft 28. The shaft 28 is rotatably supported by theplates 6 and 7, coaxially with the latter.

The lateral forces imparted by the cam 27 to each roller 26 aretransmitted to respective tracks 29 formed on the inside surface of theannular body 8, by way of rollers 30 coaxial with the respective roller26 and connected to the latter by short shafts on each side of theroller 26.

The hydraulic motor 2, the operation of which is described in thepreviously mentioned US. Pat. No. 3,583,286 is particularly suitable foruse as an electrohydraulic motor for the control of movement of memberssuch as slides, worktables and tool-carrying turrets in machine toolshaving numerical control.

In fact the hydraulic motor 2 has a very low moment of inertia, sinceits rotating parts are formed solely by the shaft 28 and the cam 27.Consequently the hydraulic motor 2 is capable of high frequencies ofoperation and high dynamic accelleration, and has good stability.Moreover, since lateral forces on the pistons 11 due to the contactbetween the rollers 26 and the cam 27 are discharged on the tracks 29through the rollers 30, the hydraulic motor 2 operates with very regularrotation, even at a very low rotational speeds.

The hydraulic motor 2 is controlled by the electric pilot motor 3. Anexternal bracket 31 is secured to the plate 7 and rigidly supports theouter casing of the electric motor 3. The electric motor 3 has an outputshaft 32 disposed parallel to the shaft 28. The shaft 32 projects fromthe electric motor 3 on the side of the latter which faces the plate 7of the hydraulic motor 2 and carries at its outer end, externally of theelectric motor 3, a toothed wheel 33. The toothed wheel 33 meshes with atoothed wheel 34 fitted on one end of a shaft 35 coaxial with the shaft28.

The shaft 35 is mounted for rotation and axial sliding movement inside acylindrical bore 36 formed in one end of the shaft 28, through theinterposition of a circulating ball bearing 37. The bearing 37 ismaintained inside the cylindrical bore 36 by a spacer 38 which is inturn retained by a metal ring 39 screwed into the open end of the bore36.

A thrust bearing 40 is located at the blind end of the bore 36 throughthe interposition of a small plate 410. The thrust bearing 40 supports asmall plate 41. A helical spring 42 is interposed between the smallplate 41 and the inner end of the shaft 35. Inside the shaft 28 a screwis threaded coaxially into the shaft 28 and is adjustable from theoutside. The screw 90 cooperates with the small plate 41a for adjustmentof the axial position of the bearing 40 inside the bore 36, in order tovary the pre-loading of the spring 42.

The outer annular end surface of the retaining ring 39, which is coaxialwith the shaft 35, is formed with a frontal cam 43. The frontal cam 43cooperates with a roller 44 mounted on a small shaft 45 mounted on andextending radially of the toothed wheel 34.

The end of the shaft 35 connected to the toothed wheel 34 projectsbeyond the latter and cooperates with a right angled quadrantthrust-transmitting lever 46 mounted for rotation on a pin 47 carried bythe bracket 31. The lever 46 is rotatable about the pin 47 in a planepassing through the axes of the distributor valve 4 and the shaft 28 andacts as a transmitting means, cooperating on one side with said end ofthe shaft 35 and on the other with the end of a stem 48 of thedistributor valve 4. The stem 48 is therefore in contact, through thequadrant lever 46, with the rotating-translating device formed by thetoothed wheel 34 and by the shaft 35. The wheel 34, and therefore theshaft 35, is rotated at an angular speed which has a fixed relation tothat of the output shaft 32 of the electric motor 3. The toothed wheel34 and the shaft 35 occupy an axial position which is a function of theangular discrepancy existing between the said rotation-translatingdevice and the shaft 28 of the motor 2, as determined by the cooperationof the roller 44 with the frontal cam 43.

Collectively the frontal cam 43, the roller 44 the relatively smallshaft 45, the toothed wheel 34 and the shaft 35 form the differentialdevice 5.

The distributor valve 4 includes a distributor spool 49, (FIG. 2) and anexternal casing 50 having an internal coaxial cylindrical through bore51 closed at its ends by two annular end plates 52 and 53 sealinglyconnected to the casing 50.

Within the bore 51, at the opposite axial ends of the latter, twotubular sleeves 54 and 55 are located, each being a tight fit in thebore 51. Three coaxial annular bodies 56, 57 and 58 are held tightlybetween the two sleeves 54 and 55, the two bodies 56 and 57 beingidentical and placed symmetrically on either side of the body 58.

The annular body 58 has an internal diameter equal to that of thesleeves 54 and 55 and has, in its external surface, which is in contactwith the surface of the bore 51, an annular groove 59. The groove 59communicates with the outside through a radial port 60 in the externalcasing 50. The port 60 forms the inlet opening of the valve 4. On thebottom of the groove 59 some radial passages 61 of small cross-sectionare provided to put the inlet port of the valve 4 into communicationwith the bore 62 extending through the sleeves 54 and 55 and the annularbodies 56, 57 and 58, in which the distributor spool 49 slides.

The annular bodies 56 and 57 have an internal diameter which is largerthan that of the annular body 58 and the tubular sleeves 54 and 55, thebodies 56 and 57 delimiting with their inner surfaces respectivecylindrical chambers 63 and 63a. The annular bodies 57 and 56 have ontheir outside surfaces respective annular grooves 64 and 65 respectivelyin communication, through radial passages 66 of small section in theannular bodies 56 and 57, with the cylindrical chambers 63 and 63arespectively. The grooves 64 and 6S communicate with respective conduits67 and 68 within the external casing 50 of the valve 4. The conduits 67and 68 communicate, through a laminar shock-absorbing plate 69 connectedto the valve 4, with the conduits 22 and 23 respectively of thehydraulic motor 2. The plate 69 is variable in the sense that it can bereplaced by different plates 69.

The distributor valve 4, is positioned with the axis perpendicular tothe axis of the shaft 28 of the hydraulic motor 2 and is supported bythe plate 7 of the latter with the interposition of the plate 69. Thelatter is sandwiched tightly between the plate 7 and the external casing50 of the distributor valve 4 and is immediately replaceable in theevent of damage or failure. On the inside of the external casing 50, atthe respective parts of the cylindrical bore 51 in which the tubularsleeves 54 and 55 are located, there are formed two grooves 70 and 71.The grooves 70 and 71 communicate with the bore 62 through substantiallyradial ducts 72 and 73 in the walls of the tubular sleeves 54 and 55respectively. The grooves 70 and 71 additionally communicate withexhaust outlets (not illustrated) which open into the outside of thevalve casing 50.

The distributor spool 49, which is slidable within the bore 62, has, inaddition to the stem 48, two pistons 74 and 75 each having across-section equal to that of the bore 62 and disposed, when at rest,in correspondence with the chambers 63 and 63a respectively. The axiallength of the pistons 74 and 75 is substantially equal to that of therespective chambers 63 and 63a.

The distributor spool 49 is supported for sliding movement in the bore62 by two cylindrical lands 76 and 77, formed integrally with the stem48. The lands 76 and 77 are slidable within the bores of the tubularsleeves 54 and 55 respectively.

The two pistons 74 and 75 and the two lands 76 and 77 define betweenthem, with the surface of the bore 62, three annular chambers 78, 79 and80.

In the previously mentioned position of rest, the chamber 78 is limitedexternally by the annular body 58 and communicates with the outsidethrough the passages 61, the groove 59 and the port 60. The annularchambers 79 and 80 communicate at the same time, through the ducts. 72and 73 respectively, with the grooves 70 and 71, and therefore, with theoutside through the exhaust outlets (not shown).

The stem 48 passes through an opening in the end plate 53, which isprovided with a suitable packing, and is rounded at its free end, where,as previously stated, it cooperates with the lever 46. The distributorspool 49 bears resiliently against the end plate 52 by way of a helicalspring 81 and a small cup 82. The cup 82 bears against the end of ascrew 83 coaxial with the casing 50 and passing through the end plate 52for control externally of the casing 50.

The electro-hydraulic motor operates as follows:

The electric stepping motor 3 receives incoming digital electricalimpulses and in response to them causes angular stepwise rotations ofthe shaft 32 and of the toothed wheel 33. The latter, meshing with thetoothed wheel 34, causes rotation of the wheel 34 with a transmissionratio which, in this example, is 1 to 5'.

The rotation of the toothed wheel 34 and, therefore, of the shaft 35,causes rolling of the roller 44 on the frontal cam 43 and, therefore, anaxial displacement of the shaft 35. The axial translation of the shaft35 is transmitted, through the quadrant lever 46, to the stem 48 of thedistributor spool 49 of the distributor valve 4.

The distributor spool 49 is initially in the condition of rest, with thepistons 74 and 75 aligned with the annular bodies 56 and 57. In thisposition hydraulic fluid supplied to the port 60 enters the groove 59and flows through the passages 61 into the chamber 78. The fluid cannot,however, enter the chambers 63 and 63a as the chamber 78 is closed atboth ends by the pistons 74 and 75.

Upon any axial displacement of the shaft 35 the stem 48 is displacedaxially by the lever 46. Upon shifting of the stem 48 the pistons 74 and75 are shifted axially so that the annular chamber 78 is put incommunication with the chamber 63 or the chamber 63a. Fluid supplied tothe port 60 enters the conduit 67 or 68 and after passing through theplate 69, enters the conduit 22 or 23 respectively to drive thehydraulic motor 2 in with the annular chamber 78. Fluid exhausted fromthe said chamber 63 or 630 reaches the outside of the valve 4 throughthe conduit 72 or 73, the groove or 71 and one of the exhaust outlets ofthe distributor valve 4.

The hydraulic motor 2 therefore rotates in one sense or in the otheraccording to the direction of rotation of the shaft 32 of the electricmotor 3. The rotation of the shaft 28 of the hydraulic motor 2 causesrotation of the frontal cam 43 carried by the said shaft 28 whichrotates in the same sense as the toothed wheel 34.

The initial relative angular displacement between the toothed wheel 34and the shaft 28 of the hydraulic motor 2 tends to diminish by theeffect of the counterreaction due to the differential device 5. Thelatter controls the opening of the ports of the valve 4 and thereforecontrols the supply of working fluid to the hydraulic motor 2 so as toannul the phase discrepancy existing between the toothed wheel 34 andthe shaft 28.

Two advantages are obtained by placing the distributor valve 4transversely to the axis of the shaft 28 of the hydraulic motor 2 andnot parallel to it. Firstly, the external casing 50 of the valve 4 canrest on the exterior of the hydraulic motor 2 and can be supported bythe latter in such a way as to permit hydraulic inter-connection of thevalve 4 and the motor 2 without using intermediate conduits, butdirectly through the plate 69, thus minimizing the quantity of workingfluid between the valve 4 and the hydraulic motor 2, which wouldotherwise impede accurate control of the hydraulic motor 2.

Secondly, by virtue of the transverse disposition of the distributorvalve 4, it is possible to adopt as differential device 5 thecombination of the frontal cam 43, the roller 44 the small shaft 45, thetoothed wheel 34 and the shaft 35. The differential device 5 acts on thestem 48 of the distributor valve 4 through the interposition of thequadrant lever 46, and consequently can impart only a movement oftranslation and not a rotational movement. In this way it is possible toprolong considerably the life of the distributor valve 4 since it doesnot rotate.

Regulation of the axial position of the distributor valve spool 49 iseffected without jumps and without vibrations since the servo-controlmechanism, including the shaft 35, the toothed wheel 34, the roller 44,the frontal cam 43, the distributor spool 49 and the lever 46, iscompressed between the opposing springs 42 and 81, which effectivelyeliminate wear-producing play among the components of said mechanism.The springs 42 and 81 are, moreover, pre-loaded and their preloading canbe regulated from the outside through the action of the screws 83 and 90respectively.

With regard to the elements forming the differential device 5, the useof the frontal cam 43 affords considerable advantages compared with theuse of the mating screw elements generally used for the same purpose inknown electro-hydraulic motors: the cam 43 is easily replaceable andeasily and economically manufactured, and the overall gain of the wholesystem is readily variable by simply replacing the frontal cam 43 byanother frontal cam having a different profile. Finally, the efficiencyof the whole system is high since frictions are minimized by the use ofball bearings and ofa circulating ball bearing in the differentialdevice.

it will be appreciated that details of practical embodiments of theinvention can be widely varied from the illustrated example, withoutdeparting from the scope of this invention.

We claim:

1. An electro-hydraulic motor of the type comprising a first steppingelectric pilot motor and a rotary hydraulic power motor havingrespective shafts, a distributor valve having a movable stem whichregulates the supply of working fluid to the hydraulic motor, and adifferential device connected to said movable stem of the distributorvalve to displace the latter proportionally to the angular discrepancyexisting between the shaft of the electric pilot motor and the shaft ofthe hydraulic motor, wherein the improvement consists in: thedifferential device comprising a first toothed wheel coupled directly tothe shaft of the electric pilot motor, a shaft of the differentialdevice coaxial with the shaft of the hydraulic motor and on which asecond toothed wheel meshing with the first toothed wheel is mounted:means supporting said shaft of the differential device for rotation andaxial displacement relative to the hydraulic motor shaft and facing thesecond toothed wheel, and cam-follower means interposed between thefrontal cam and said toothed wheel and supported by the latter; thedistributor valve being disposed transversely of the shaft of thehydraulic motor and connected hydraulically to the hydraulic motor;transmitting means interconnecting said shaft of the differential deviceand the stem of the distributor valve and transforming axial movements,in either sense, of the said shaft of the differential device intocorresponding axial movements of said stem; and elastic means acting onthe said shaft of the differential device and said stem to maintain themalways in contact with said transmitting means.

2. Motor according to claim 1, in which the shaft of the hydraulic motorhas a cylindrical bore and includ ing a circulating ball bearing in saidbore supporting the shaft of the differential device radially.

3. Motor according to claim 2, including a thrust bearing located at theaxially inner end of said bore in the hydraulic motor shaft and a springlocated between such thrust bearing and the shaft of the differentialdevice.

4, Motor according to claim 2, wherein the axially outer end of saidcylindrical bore is threaded and carries a correspondingly threadedtubular member arranged coaxially with the shaft of the differentialdevice, the said frontal cam being formed integrally on the axiallyouter end of said member.

5. Motor according to claim 1, wherein the cam-follower means comprise arotatable roller the axis of which extends radially with respect to thesecond toothed wheel and a small shaft carrying said roller andsupported by the said toothed wheel on the surface of the latter facingthe frontal cam.

6. Motor according to claim 1, wherein the transmitting means comprise aright angled lever pivotally mounted on the hydraulic motor for rockingmovement about an axis perpendicular to the plane containing the axes ofthe shaft of the differential device and the stem of the distributorvalve, the said lever being in contact on one side with the outer end ofsaid shaft of the differential device and on the other side with theouter end of the said stem outside the distributor valve.

7. Motor according to claim 3, wherein the transmitting means comprise aright-angled lever pivotally mounted on the hydraulic motor for rockingmovement about an axis perpendicular to the plane containing the axes ofthe shaft of the differential device and the stem of the distributorvalve, the said lever being in contact on one side with the outer end ofsaid shaft of the differential device and on the other side with theouter end of the said stem outside the distributor valve, and whereinthe distributor valve has an end plate, the elastic means formaintaining the shaft of the differential device and the stern of thedistributor valve always in contact with the right-angled levercomprising two helical springs the first of which is the spring locatedwithin said cylindrical bore of the hydraulic motor shaft between theinner end of the shaft of the differential device and the thrustbearing, and the second of which is interposed between the end of thedistributor valve stem opposite to that cooperating with theright-angled lever and the end plate of the distributor valve.

latter in cooperation with the thrust bearing supporting the shaft ofthe differential device to vary the axial position of the said bearinginside the bore of the hydraulic motor shaft, means being provided foradjusting the said first and second screws from the outside.

10. Motor according to claim 1, including a replaceable shock-absorbingmember through which the distributor valve is connected to the hydraulicmotor.

1. An electro-hydraulic motor of the type comprising a first stepping electric pilot motor and a rotary hydraulic power motor having respective shafts, a distributor valve having a movable stem which regulates the supply of working fluid to the hydraulic motor, and a differential device connected to said movable stem of the distributor valve to displace the latter proportionally to the angular discrepancy existing between the shaft of the electric pilot motor and the shaft of the hydraulic motor, wherein the improvement consists in: the differential device comprising a first toothed wheel coupled directly to the shaft of the electric pilot motor, a shaft of the differential device coaxial with the shaft of the hydraulic motor and on which a second toothed wheel meshing with the first toothed wheel is mounted: means supporting said shaft of the differential device for rotation and axial displacement relative to the hydraulic motor shaft and facing the second toothed wheel, and cam-follower means interposed between the frontal cam and said toothed wheel and supported by the latter; the distributor valve being disposed transversely of the shaft of the hydraulic motor and connected hydraulically to the hydraulic motor; transmitting means interconnecting said shaft of the differential device and the stem of the distributor valve and transforming axial movements, in either sense, of the said shaft of the differential device into corresponding axial movements of said stem; and elastic means acting on the said shaft of the differential device and said stem to maintain them always in contact with said transmitting means.
 2. Motor according to claim 1, in which the shaft of the hydraulic motor has a cylindrical bore and including a circulating ball bearing in said bore supporting the shaft of the differential device radially.
 3. Motor according to claim 2, including a thrust bearing located at the axially inner end of said bore in the hydraulic motor shaft and a spring located between such thrust bearing and the shaft of the differential device.
 4. Motor according to claim 2, wherein the axially outer end of said cylindrical bore is threaded and carries a correspondingly threaded tubular member arranged coaxially with the shaft of the differential device, the said frontal cam being formed integrally on the axially outer end of said member.
 5. Motor according to claim 1, wherein the cam-follower means comprise a rotatable roller the axis of which extends radially with respect to the second toothed wheel and a small shaft carrying said roller and supported by the said toothed wheel on the surface of the latter facing the frontal cam.
 6. Motor according to claim 1, wherein the transmitting means comprise a right angled lever pivotally mounted on the hydraulic motor for rocking movement about an axis perpendicular to the plane containing the axes of the shaft of the differential device and the stem of the distributor valve, the said lever being in contact on one side with the outer end of said shaft of the differential device and on the other side with the outer end of the said stem outside the distributor valve.
 7. Motor according to claim 3, wherein the transmitting means comprise a right-angled lever pivotally mounted on the hydraulic motor for rocking movement about an axis perpendicular to the plane containing the axes of the shaft of the differential device and the stem of the distributor valve, the said Lever being in contact on one side with the outer end of said shaft of the differential device and on the other side with the outer end of the said stem outside the distributor valve, and wherein the distributor valve has an end plate, the elastic means for maintaining the shaft of the differential device and the stem of the distributor valve always in contact with the right-angled lever comprising two helical springs the first of which is the spring located within said cylindrical bore of the hydraulic motor shaft between the inner end of the shaft of the differential device and the thrust bearing, and the second of which is interposed between the end of the distributor valve stem opposite to that cooperating with the right-angled lever and the end plate of the distributor valve.
 8. Motor according to claim 7, including presettable means varying the pre-loading of the said springs.
 9. Motor according to claim 8, wherein said presettable means varying the pre-loading of the springs comprise a first screw passing through said end plate of the distributor valve a plate through which said first screw, cooperates with an end of the spring disposed inside the valve and a second screw disposed coaxially with the shaft of the hydraulic motor and mounted inside the latter in cooperation with the thrust bearing supporting the shaft of the differential device to vary the axial position of the said bearing inside the bore of the hydraulic motor shaft, means being provided for adjusting the said first and second screws from the outside.
 10. Motor according to claim 1, including a replaceable shock-absorbing member through which the distributor valve is connected to the hydraulic motor. 